The doping distribution in the multiplication zone of n+-p-π-p+ structured linear avalanche photodiode (APD) based on standard CMOS process greatly determines the device performance. The influences of implanting dose and the depth of its peak concentration of the p-layer on device characteristics are simulated using Silvaco. The simulation results show that, at a given gain of 50, the optimized doping dose of P layer is 1.82×1012/cm2 with depth of peak concentration 2.1 μm. Under optimized conditions, the reverse bias voltage is 73.1 V, the excess noise factor is 4.59, and the excess noise index is 0.34~0.45 (λ=800 nm),which are better than those reported. The performance of the APD may be further improved through process optimization.